1. Technical Field of the Invention
This invention relates to a fail-safe device for use in a heating appliance, such as an electric oven and a microwave oven, and more particularly it relates to a fail-safe device which includes an electronic control circuit for controlling a main circuit of the heating apparatus and a so-called mechanical latch for unlocking an actuator lever fo a fail-safe switch responsive to the movement of a door and for turning on a switch in the main circuit, upon mechanical actuation of a cook start button for starting cooking, thereby rendering a control system for the heating apparatus operative only when the user mechanically depressing the cook switch.
2. Background Prior Art
The heating appliance of the above described type is in danger that it may be self-triggered due to incoming noise, a surge of lightning, electrostatic noise, momentary power failure, etc. and become operative when it is not desirable.
It is most dangerous for the heating appliance of the above type that it may start heating inadvertently notwithstanding no start instruction has been applied to a control system for heating. Under these circumstances, the temperatures of the appliance and the door rise extraordinally and the user may burn himself on these portions or the appliance itself may catch fire and burn surrounding inflammables such as a curtain.
In addition, non-loaded heating in a microwave oven results in increasing the quantity of microwave radiations leaking from the periphery of the door and impairing greatly the lives of power unit components such as a magnetron.
With the recent remarkable development of the electronic control circuit technology using microcomputers, microcomputer-based appliances are growing in number. Even though in those appliances electronic control circuits and softwares stored in the microcomputer are designed with a sufficient allowance or provision for coping with a surge of lightning, a momentary power failure and so forth, the microcomputer sometimes performs a faulty operation due to such surge of lightning or momentary power failure or a drop in power supply voltage, for instance.
For example, there is the possibility of burning the interior of the heating chamber, the appliance itself, other equipment or furniture when the appliance inadvertently becomes operative during the night time.
To prevent the above accident beforehand, a mechanical power switch is additionally provided. However, this switch results in increasing the number of actuations, impairing the convenience of the user of the appliance, and if the power switch is kept on, causing the above-mentioned accident. The mechanical latch method is effective for those reasons but it is not available for use in a microcomputer-controlled high frequency heating appliance.
In recent years, a mechanical latch method has been proposed in lieu of an electromagnetic relay method in an attempt to simplify not only circuit structure but also mechanical control. With the former method, a lever operatively interlocked with a door is unlocked to turn on a switch for a main circuit upon actuation of a cook start button. With the latter method, an electromagnetic relay coil is energized to turn on a main circuit for a relay main circuit upon depression of the cook start button.
The reason why the mechanical latch method is not available for use in the microcomputer-controlled microwave oven is due to the fact that the mechanical latch switch serves as a cook start switch per se.
The microcomputer, on the other hand, demands a cook start signal switch. Although it might be contemplated to stack this switch on the mechanical latch switch, this approach will face the following difficulties.
Once the cook start button is depressed, the cook start signal switch is turned on and remains in that state. If the cook start button is depressed before selection of a desired kind of cooking, i.e. one of such cooking modes as re-heating with a high output level, cooking with a high output level, defrosting with a low output level, simmering with high and low output levels, heater cooking with top and bottom heaters, cooking with the top heater only for browning, cooking with the bottom heater only, fermentation with low output level heater control, heater cooking with hot air circulation, etc., in the case of a high frequency heating appliance having the electric heater built therein, the microcomputer causes the various mechanisms of the appliance to operate as long as the cook start button is in depressed position. Then, as a cook selection switch is manipulated, various switches are switched in live or energized condition, thus detracting considerably from the serviceable lives and dependability of the switches.